The integrity of the heat exchanger tubes in steam generators is of major concern to the nuclear industry for both performance reasons and economic considerations. Periodic inspections of these tubes must be conducted to look for degradation of the tubing, such as denting, pittinq, cracking which sometimes occurs in the tubesheet and support plate regions of the generator. As such nuclear steam generators become older, it is becoming increasingly important to develop new techniques of in service inspection for detection of tube degradation.
Eddy current probes and ultrasonic probes for in service inspection of such tubes are known in the prior art. Originally, eddy current in service inspection techniques were developed to detect wall thinning of the heat exchanger tubes. Such eddy current testing was based on the use of alternating current frequency instrumentation in conjunction with electric coil probes. The alternating current conducted through the probe coil created a time varying magnetic field which is turn induced eddy currents in the metallic walls of the heat exchanger tubes. These eddy currents in turn produced counter-magnetic fields that impeded the time varying magnetic field generated by the probe coil. Discontinuities caused by flaws in the metallic walls of the tubes tend to resist the eddy currents, which in turn lowers the impedance that the magnetic fields associated with the eddy currents imposes on the magnetic field of the probe coil. Thus, flaws could be detected by monitoring impedance changes in the coil of the eddy current probe.
As this test procedure evolved, multiple frequency, computer controlled instrumentation systems were used which could be connected to any number of special purpose eddy current probes each, of which was designed to detect a specific type of degradation. However, even with such improvements, eddy current inspection techniques could not detect some types of degradation with any reliability, and in some cases it was difficult or impossible to discriminate between signals produced by combinations of different types of degradation which may occur in a single location in the tube.
As a result, ultrasonic inspection systems have been developed which have been found to more accurately indicate what type of degradation might be present within a particular tube. Like eddy current test procedures, such ultrasonic systems have evolved since their introduction so that they now use sophisticated multi-channel, multi-transducer electronic systems capable of resolving different types of flaws. Such ultrasonic systems take a much longer time to conduct an inspection than eddy current inspection systems, although they do have the advantage of being especially effective in resolving flaws where eddy current testing is the most limited. The complimentary character of the ultrasonic and eddy current inspection techniques has led to the concurrent use of both methods in a combination probe, an example of which is disclosed in U.S. patent application Ser. No. 079,860 filed July 7, 1987, now U.S. Pat. No. 4,856,337, by Warren Junker et al. and assigned to the Westinghouse Electric Corporation.
Ultrasonic testing techniques in a preferred usage require a couplant fluid such as water between the ultrasonic probe and the wall of the tube being tested. The couplant fluid increases the sensitivity of the probe in detecting flaws. In order to run a test, it is necessary to helically move the ultrasonic probe while also supplying the couplant medium around the probe during the testing procedure. In known ultrasonic systems, water couplant was injected into the tube around the ultrasonic probe by way of the same flexible drive shaft that was used to position and helically move the probe within the tube. Keeping the water couplant around the probe was accomplished by a barrel seal located around the flexible drive shaft. In such a known system, the transducer of the probe was moved circumferentially and axially within the tube by means of a screw thread, and the position of the probe relative to the tube wall was determined by means of a rotary encoder connected to the screw thread.
The drive system in this prior art system which rotated the flexible drive shaft was located at and mounted to the tubesheet of the steam generator. From the tubesheet, a flexible shaft was rotatably driven to helically move the ultrasonic probe within the steam generator tubing. However, this type system was limited in that the probe could only be extended into the tubes to a height of only about two meters above the tubesheet. Since the legs of the heat exchanger tubes are about 10 meters long, much of the tubing could not be inspected. Additionally, this system was cumbersome to move from tube to tube within the tubesheet.
To overcome these limitations, a drive system was developed that incorporated a miniature drive motor coupled to a rotary encoder device that could be inserted into the tube along with the probe body. This arrangement circumvented the previous limitation on probe height above the tubesheet, and the probe could be positioned anywhere up to the start of the U-bend of the tube. This drive system was also an improvement over the tubesheet mounted drive system in that the probe and its drive system could be easily moved from tube to tube with the use of known robots.
This drive system consists of a miniature motor coupled in tandem with a gear box and a screw mechanism that was mounted at the probe end which the probe itself was attached. This system also included an expandable, fluid actuated bladder which circumscribed the probe body for maintaining the position of the probe within the tube, and a slip ring arrangement that allowed electrical power to be conducted to the motor and to the probe. Unfortunately, this drive system has not proven to be entirely satisfactory due in part to the power limitation associated with the small size of the motor. It also has proven difficult to completely seal the electrical components such as the motor and slip ring arrangement from the couplant water. Finally, it has proven to be very difficult to effectively prevent the electrical wires between the probe and the slip ring arrangement from binding or tangling during the operation of the device, as a result of the distance changes between the probe and the slip ring as the screw mechanism axially moves the probe relative to the slip ring arrangement. In an attempt to solve this last problem, the wires were loosely coiled in an orifice within the cartridge to provide the required slack. However, the scan length was still limited to only about 2.5 cm with each location of the probe by the bladder. Thus, it was necessary to relocate the entire probe assembly many times to inspect even a decimeter of tubing. Since ultrasonic testing is in itself a slow process, the process was unreasonably lengthened by having to relocate the probe assembly so many times to do an inspection of any significant length of tubing.
Clearly, there is a need for a drive system that can helically move an inspection probe assembly having an ultrasonic probe over a greater stroke distance than prior art systems, and which has a sufficient torque to rotatably drive the probe while maintaining the electrical components safe from couplant water. Ideally, the drive system should allow the probe assembly to be easily moved from tube to tube, and should be highly reliable in operation.